Modulation circuit



' May '27, 1930 w. SCHAFFER 1,760,2

MODULATION CIRCUIT Filed- Feb. 14, 1924 ill ' anoemmz WALTER SLHRFFER til Patented May 27, 1930 entree VJ'ALTEE SCHAFFER, 0E BERLIN, GERMANY, ASSIGNOR TO GESELLSCHAFT Fl'jR DEAHT- LOSE TELEGRAPHIE M. B. EL, OF BER-LIN, GERMANY, A CGRPORATION OF GEBILAIZ'Y LIODULATION CIRCUIT Application filed February 14, 1524:, Serial No. 892,628, and in Germany February 16, 1923.

My invention relates to radio signalling systems and particularly to such systems containing vacuum tube oscillation generators arranged for modulation.

An object of my invention is to provide a vacuum tube oscillation generator and a vacuum tube modulatorcombined with means 7 for preventing the feed back of the generated radio frequency oscillations to the modulator triode.

Another object of my invention is to provide a vacuum tube oscillation generating means, a modulating vacuum tube connected in the grid circuit thereof and means combined with the vacuum tube modulator for preventing undesired oscillations therein.

Vacuum tube oscillation generating systems have been constructed in which the amplitude of oscillation is controlled by a vacuum tube resistance connected in the grid circuit. In this system the effective resistance. of the controlling vacuum tube is controlled by the application of potential to its grid which in turn controls the potential of the grid of the oscillator tube and thereby the amplitude of oscillation. It has however, been found that this control valve or modulat-or tube is very sensitive to radio frequency current and that the application of radio frequency potentials thereto may cause undesirable effects in the modulator tube, producing undesired oscillations.

Previously devised measures for protecting the modulator tube from radio frequency have consisted of a condenser usually connected in parallel to the modulator tube or of a radio frequency choke coil inserted ahead of the modulator tube. However these devices have not proved to be sufficient forprotection of the modulator tube since the'protecting condenser must be small in order not to short circuit the speech currents and since a radio frequency choke coil does not prevent trouble from small residual radio frequency currents which are not choked out.

My invention provides means for protecting the modulator tube completely from the radio frequency currents by connecting a tuned circuit in parallel to the modulator tube. This device consists of a condenser and an inductance connected in series and tuned to the generation frequency, thereby forming a series resonant circuit having a low impedance to the generation frequency. This circuit in parallel to the modulator tube bypasses the radio frequency and results in the application of a zero or negligable voltage only to the modulator tube.

Other objects and structural details of my invention will be apparent from the follow ing description when read in connection with the accompanying drawing wherein Fig. '1 shows a preferred embodiment of my invention and Fig. 2 shows an alternativeembodiment.

Referringto 1 an antenna circuit A is energized by oscillations from a vacuum tube generator B having a plate circuit coil D and a grid circuit coil E coupled thereto. A modulator tube F is connected in the grid circuit between coil E and the tube B. This tube serves as a variable grid leak under the control of a vibratory current input through the transformer G. Thus theoutput circuitof the modulator tube may be considered as the modulation effecting circuit of the particular modulating means employed. The tuned protective device of my invention, consists of the condenser C and. the inductance L. These form a series resonant circuit and are tuned to the frequency of oscillation of the tube B and its circuits. They serve to bypass'the feed back energy past the tubeF and prevent the application of energy at the generated frequency to this tube. This protection is obtained because of the fact that the inductance L andcapacity C in combinatmnhave a very low impedance to the radio frequency energy, and therefore, a substantially zero radio frequency energy only is impressed across the terminals of thetube F. 'The presence of the condenser C simultaneously-pre vents short circuiting of the grid bias poten v tial.

Another similar protecting arrangement may also be connected directly :to the plate and filament of the valves. In this conned tion it is of no consequence whether the modulating tube, that is, the controlling as in the previous embodiment, but is connected in parallel to the grid coil instead of in series. In this embodiment a radio frequency choke coil 3 serves to prevent access of most of the grid circuit oscillations to the modulator tube F and such small oscillations as do pass the coil 3 are bypassed by the series resonant circuit consisting of the inductance L and the capacitance C as in the embodiment of Fig. 1.

Bythe system of my invention I am enabled to obtain grid modulation of a vacuum tube generator and to prevent the interfering effects of the generated radiofrequency upon the modulator tube, thereby providing more certain, stable, and accurate operation. l/Vhile I have shown but two embodiments of my invention in the drawings, it is capable of various changes and modifications therefrom without departing from the spirit thereof and it is desired therefore, thatonly such limitations shall be imposed thereon as are inclicated in the appended claims or required by the prior art.

Having described my invention, what I claim is 1. A modulated radio frequency generating system comprising a triode oscillation generator, means for controlling the grid bias thereof at signal frequency and means including a series resonant circuit tuned to the oscillation frequency connected in parallel with the bias control means for preventing influence upon'said bias control means by the generated oscillations.

2. A radio system comprising a vacuum tube oscillation generator having grid and plate circuits, a modulator triode connected in the grid circuit of said generator and adapted to control the generator grid bias, means for varying the impedance of the modulator triode, and means for stabilizing the modulator triode comprising a series reso nant circuit in shunt thereto and tuned to the oscillation frequency of saidgenerator.

3. Ina radio device, a vacuum tube 'enerator, a grid condenser associated therewith, a grid leak for said condenser, said grid leak comprising a space current path, an inductance arranged in parallel with said grid leak and in series with said condenser, said condenser and inductance being proportioned so as to form a series resonant circuit tuned to the generated frequency,

4. A modulated radio frequency generating system comprising a triode oscillation generator, means associated with said oscillation generator for controlling the grid bias thereof signal frequency comprising a variable grid leak and grid condenser device and means including a series resonant circuit tuned to the oscillation frequency connected in parallel with the bias control means for preventng influence upon said bias control means by the generated oscillations.

5. A radio system comprising a vacuum tube oscillation generator having grid and plate circuits, an antenna coupled thereto, a grid condenser and grid leak connected in the grid circuit of said generator and adapted to control the generator grid bias, means for varying the impedance of said grid leak and means for stabilizing said grid leak comprisin a series resonant circuit in shunt thereto and tuned to the oscillation frequency of said generator.

N ALTER SCHAFFER. 

